Alkylene oxide separation systems, methods, and apparatuses

ABSTRACT

A propylene oxide separation system that comprises a distillation column, a decanter, and water wash system. The distillation column is configured to receive a crude propylene oxide stream, discharge an impurity stream that comprises methanol and water, and discharge a bottoms stream that comprises a majority of the propylene oxide entering in the crude propylene oxide stream. The decanter is configured to receive at least a portion of the impurity stream and a hydrocarbon solvent to provide for formation in the decanter of an organic phase and an aqueous phase. The organic phase comprises propylene oxide and hydrocarbon solvent, and is sent to the distillation column. The aqueous phase comprises a majority weight percent of the methanol and the water entering in the impurity stream. The water wash system is configured to receive and purge the aqueous phase from the propylene oxide separation system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit and priority of U.S. Provisional PatentApplication No. 61/859,549 filed on Jul. 29, 2013, which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a process for the purification andrecovery of propylene oxide which is formed from epoxidation ofpropylene with hydroperoxides derived from oxidation of isobutane, ethylbenzene or cumene. In particular, the process improves the separation oflight aldehydes, such as formaldehyde and acetaldehyde, from propyleneoxide.

BACKGROUND OF THE INVENTION

Approximately 14.5 billion pounds of propylene oxide are produced everyyear. Propylene oxide has many uses. Between 60 and 70% of all propyleneoxide is converted to polyether polyols for the production ofpolyurethane plastics. About 20% of propylene oxide is hydrolyzed intopropylene glycol, via a process which is accelerated either by thermalreaction or by acid or base catalysis. Other major products arepolypropylene glycol, propylene glycols ethers, and propylene carbonate.To produce these end products, propylene oxide free of impurities isneeded.

Methods of producing alkylene oxides including propylene oxide involvehydrochlorination and epoxidation of its corresponding olefins. Theoxidates used in the epoxidation processes are derived from tertiary orsecondary hydrocarbons by direct oxidation with molecular oxygen; hence,they contain oxygenate impurities and precursors. Additional oxygenateimpurities are also generated in the step of epoxidation of olefins.Crude alkylene oxides, such as propylene oxide, particularly thoseproduced from epoxidation with hydrocarbon oxidates contain asignificant amount of oxygenated impurities difficult to separate fromalkylene oxides. The impurities generally include water, acids,alcohols, aldehydes, ketones and esters. A need exists for continuedimprovement of systems and methods for separating propylene oxide fromthese impurity constituents of effluent streams of various methods ofproducing propylene oxide.

U.S. Pat. No. 3,338,800 teaches extractive distillation of alkyleneoxides having from 3 to 18 carbon atoms with a paraffin or paraffinnaphtha solvent. More particularly, this patent suggests that oxygenatedimpurities boiling within 5° C. of the alkylene oxide may be separatedby extractive distillation using acyclic paraffinic hydrocarbons assolvents having boiling points at least 35° C. above the boiling pointsof the said impurities. The problem addressed by this patent is thatepoxide fractions produced by the direct oxidation of ethylenicallyunsaturated compounds with molecular oxygen in the liquid phase containoxygenated impurities which, because their boiling points are similar tothe desired epoxide product, cannot be separated by conventionaldistillation techniques.

U.S. Pat. No. 3,881,996 teaches that the sequence of the fractionationsteps has a major effect on the final purity of the propylene oxideobtained, particularly with regard to aldehyde content. Substantiallyimproved results are obtained when the removal of acetaldehyde and lowerboiling materials precedes the step in which propylene oxide isseparated from propionaldehyde and higher boiling material. This resultis highly unusual and is not in accord with customary calculableperformance of fractional distillation equipment. The inventor believesthat chemical reactions may be occurring during distillation whichinterfere with the normal mass transfer steps and thereby produceanomalous results. However, the scientific reasoning is not offered.

U.S. Pat. Nos. 3,464,897 and 3,843,488 teach using hydrocarbon solventsof 8-20 carbon atoms can effective remove C5-C7 impurities frompropylene oxide in extractive distillation. U.S. Pat. No. 3,607,669teaches a method for separating propylene oxide from water by distillingthe mixture in the presence of acyclic or cyclic paraffin containing 8to 12 carbon atoms by breaking water-propylene oxide azeotrope atelevated pressure. There are many other U.S. Patents, such as U.S. Pat.Nos. 4,140,588, 5,000,825, 5,006,206, 5,116,466, 5,116,467, 5,139,622,5,145,561, 5,145,563, 5,154,803, 5,154,804, 5,160,587, 5,340,446,5,620,568, 5,958,192 and 6,559,248 that reflect use of various solventsin extractive distillation operations for propylene oxide purification.U.S. Pat. Nos. 2,550,847, 2,622,060, 3,350,417, 3,477,919, 4,691,034,4,691,035, 5,106,458 and 5,107,002 teach how to separate methyl formatefrom propylene oxide. Although these patents teach the removal ofselected propylene oxide impurities, none address removal of aldehydes,particularly formaldehyde and acetaldehyde.

U.S. Pat. No. 6,024,840 uses methanol as extractive solvent to removeacetaldehyde from propylene. However, solvent methanol itself becomesclose-boiling propylene oxide contaminant. U.S. Pat. No. 7,705,167teaches using water wash propylene oxide followed by contacting aqueousphase with hydrocarbon extractive solvent and subsequent distillation.These teachings are impractical for the existing plant improvement.Because it is difficult to recover a propylene oxide containing totalaldehydes below 50 ppm and free of formaldehyde, particularly forpropylene oxide produced from tert-butyl hydroperoxide process, it isthe objective of the present invention to provide a method applicable tothe existing plants for recovering propylene oxide in a high state ofpurity low in aldehydes without substantial loss of propylene oxideproduct.

SUMMARY OF THE INVENTION

An aspect of the invention relates to propylene oxide separation systemincluding: a distillation column configured to receive a crude propyleneoxide stream, discharge an impurity stream having methanol and water,and discharge a bottoms stream having a majority of the propylene oxideentering in the crude propylene oxide stream; a decanter configured toreceive the impurity stream and a hydrocarbon solvent to provide forformation in the decanter of an organic phase having propylene oxide andhydrocarbon solvent, and an aqueous phase comprising a majority weightpercent of the methanol and the water entering in the impurity stream;and a water wash system configured to receive and purge the aqueousphase from the propylene oxide separation system, wherein the organicphase in the decanter is sent to the distillation column.

The crude propylene oxide stream may be a propylene oxide reactoreffluent stream, such as in a propylene oxide/tert-Butanol processsystem. The distillation column may include an overhead condenser, andwherein the distillation column is configured with an overhead vaporpurge of non-condensed components from the overhead condenser. Thedecanter maybe an overhead decanter to the distillation column, andreceive the impurity stream from the overhead condenser. On the otherhand, the decanter may be a side decanter to the distillation column,and receive the impurity stream from a liquid side draw of thedistillation column. The distillation column may be a solvent-lightscolumn. The water wash system may include a mixer, such as a staticmixer, and a coalescer. Further, a solvent stripper may receive thebottoms stream from the distillation column, wherein the solventstripper discharges a solvent-stripper overhead stream having a majorityof the propylene oxide entering the solvent stripper in the bottomsstream from the distillation column, and discharges a solvent-stripperbottoms stream comprising at least a portion of the hydrocarbon solventreceived at the decanter. Additionally an extraction column may subjectthe solvent-stripper overhead stream from the solvent stripper to ahydrocarbon solvent extraction to remove impurities, wherein theextraction column purges the removed impurities having formaldehyde tothe water wash system

Another aspect of the invention relates to a method for separatingpropylene oxide from a crude propylene oxide stream in a separationsystem, the method including: feeding the crude propylene oxide streamto a distillation column; discharging an impurity stream from thedistillation column to a decanter, the impurity stream having methanoland water; feeding hydrocarbon solvent to the decanter; forming in thedecanter an organic phase including propylene oxide and hydrocarbonsolvent, and an aqueous phase having a majority weight percent of themethanol and the water fed to the decanter in the impurity stream;washing the aqueous phase with water and purging the washed aqueousphase from the separation system; and sending the organic phase to thedistillation column.

The discharging of the impurity stream may include discharging theimpurity stream to the decanter via an overhead condenser of thedistillation column, and the method further including purging a vaporstream from the overhead condenser. On the other hand, the dischargingthe impurity stream may involve discharging the impurity stream to thedecanter via a liquid side draw of the distillation column. The methodmay include: discharging a bottoms stream from the distillation column,the bottoms stream having a majority of the propylene oxide entering thedistillation column in the crude propylene oxide stream; separatingformaldehyde from the bottoms stream; and sending the formaldehyde to awater wash system performing the washing of the aqueous phase withwater.

Yet another aspect of the invention relates to a propylene oxideseparation system including: a distillation column configured to receivea processed crude propylene oxide stream, discharge an impurity streamcomprising methanol and water, and discharge a bottoms stream having amajority of the propylene oxide entering in the processed crudepropylene oxide stream; a mixer configured to mix caustic (e.g., is orhaving sodium hydroxide) with the impurity stream to give acaustic-treated impurity stream; and a backwash column configured tosubject the caustic-treated impurity stream to both an aqueousextraction and an organic extraction.

The backwash column may purge an aqueous stream having a majority amountof the methanol and the water in the impurity stream. Also, the backwashcolumn may discharge an organic stream (having hydrocarbon solvent andpropylene oxide) to the distillation column. An extraction column may bedisposed downstream of the distillation column, and purge formaldehydeto the mixer, wherein the formaldehyde is carryover from the bottomsstream of the distillation column.

The propylene oxide separation system may further include: a lightsdistillation column configured to receive a crude propylene oxidestream, remove light components, and discharge a lights-distillationcolumn bottoms stream comprising a majority of the propylene oxide fromthe crude propylene oxide stream; and a heavies distillation columnconfigured to receive the lights-distillation column bottoms stream,remove heavy components, and discharge an overhead stream comprising amajority of the propylene oxide from the lights-distillation columnbottoms stream, and wherein the overhead stream is or a portion of theprocessed crude propylene oxide stream. Alternatively, the propyleneoxide separation may include: a heavies distillation column configuredto receive a crude propylene oxide stream, remove heavy components fromthe crude propylene oxide stream, and discharge an overhead streamcomprising a majority of the propylene oxide from the crude propyleneoxide stream; and a lights distillation column configured to receive theoverhead stream, remove heavy components from the overhead stream, anddischarge a lights-distillation column bottoms stream having a majorityof the propylene oxide from the overhead stream, and wherein thelights-distillation column bottoms stream is or a portion of theprocessed crude propylene oxide stream.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims. The novel features which are believed to be characteristic ofthe invention, both as to its organization and method of operation,together with further objects and advantages will be better understoodfrom the following description when considered in connection with theaccompanying figures. It is to be expressly understood, however, thateach of the figures is provided for the purpose of illustration anddescription only and is not intended as a definition of the limits ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims, and accompanying drawings where:

FIG. 1 is a schematic block diagram of a propylene oxide separationsystem according to one embodiment.

FIG. 2 is a schematic, including a solvent-lights column, according toone embodiment, as used in a pilot plant.

FIG. 3 is a schematic of a solvent stripper column, according to oneembodiment, as used in a pilot plant.

FIG. 4 is a schematic block diagram of a propylene oxide separationsystem according to various embodiments.

FIG. 5 is a schematic block diagram of a front-end of a propylene oxideseparation system according to one embodiment.

FIG. 6 is a schematic block diagram of a back-end of a propylene oxideseparation system associated with the front-end of FIG. 5 according toone embodiment.

FIG. 7 is a schematic block diagram of another front-end of a propyleneoxide separation system according to one embodiment.

FIG. 8 is a schematic block diagram of an example of a solvent-lightscolumn system of the front-end of FIG. 7 according to one embodiment.

FIG. 9 is a schematic block diagram of another example of asolvent-lights column system of the front-end of FIG. 7 according to oneembodiment.

FIG. 10 is a schematic block diagram of a back-end of a propylene oxideseparation system associated with the front-end of FIGS. 7-9 accordingto one embodiment.

FIG. 11 is a schematic block diagram of yet another front-end of apropylene oxide separation system according to one embodiment.

FIG. 12 is a schematic block diagram of a back-end of a propylene oxideseparation system associated with the front-end of FIG. 11 according toone embodiment.

It should be understood that the various embodiments are not limited tothe arrangements and instrumentality shown in the drawings.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present disclosure may be understood more readily by reference tothe following detailed description of preferred embodiments of theinvention as well as to the examples included therein. Various streamsare discussed throughout the present disclosure as containingimpurities, which are identified below within the context of theparticular stream. Although various streams may be identified below bymore specific names, to the extent a stream is identified as containingimpurities to be removed, such stream is also an impurity stream.

One method for producing propylene oxide (PO), also known asepoxypropane, propylene epoxide, 1,2-propylene oxide, methyl oxirane,1,2-epoxypropane, propene oxide, methyl ethylene oxide, methylethyleneoxide, will now be described. First, as shown in Scheme 1, isobutane(IB), also known as 2-methylpropane, can be reacted with oxygen to formtert-butyl hydroperoxide (TBHP), also known as2-Methylpropane-2-peroxol.

Subsequently, as shown in Scheme 2, propylene, also known as propene,can be reacted with TBHP in the presence of a catalyst to form PO andtert-Butanol (TBA), also known as 2-methyl-2-propanol.

Since this method produces both PO and TBA it shall be referred to asthe PO/TBA process.

The PO/TBA process can also yield a variety of unwanted side products.Without wishing to be bound by theory, non-selective reactions can takeplace to produce the impurities. Such non-selective reactions caninclude, but are not limited to the reactions depicted in Schemes 3-6.

Acetaldehyde can also be formed in the PO/TBA process. A possiblemechanism for the formation of acetaldehyde is shown in Scheme 7.

The concentrations of these impurities that end up in a crude PO streamfrom a PO/TBA process can vary.

Methyl formate can be present in an amount within a range having a lowerlimit and/or an upper limit, each expressed as a weight percentage ofthe total composition of a crude PO stream from a PO/TBA process. Therange can include or exclude the lower limit and/or the upper limit. Themethyl formate lower limit and/or upper limit can be selected from 0,0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12,0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, 0.21, 0.22, 0.23, 0.24,0.25, 0.26, 0.27, 0.28, 0.29, 0.3, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36,0.37, 0.38, 0.39, 0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48,0.49, 0.5, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.6,0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.7, 0.71, 0.72,0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.8, 0.81, 0.82, 0.83, 0.84,0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96,0.97, 0.98, 0.99, 1, 2, 3, 4, 5, and 10 weight percent. For examplemethyl formate can be present in an amount of greater than 0.06 weightpercent of the total composition of a crude PO stream from a PO/TBAprocess.

Methanol can be present in an amount within a range having a lower limitand/or an upper limit, each expressed as a weight percentage of thetotal composition of a crude PO stream from a PO/TBA process. The rangecan include or exclude the lower limit and/or the upper limit. Themethanol lower limit and/or upper limit can be selected from 0, 0.001,0.002, 0.003, 0.0031, 0.0032, 0.0033, 0.0034, 0.0035, 0.0036, 0.0037,0.0038, 0.0039, 0.0139, 0.0239, 0.0339, 0.0439, 0.0539, 0.0639, 0.0739,0.0839, 0.0939, 0.1039, 0.1049, 0.1059, 0.1069, 0.1079, 0.1089, 0.1099,0.1109, 0.1119, 0.1129, 0.1139, 0.1149, 0.1159, 0.116, 0.1161, 0.1162,0.1163, 0.1164, 0.1165, 0.1166, 0.1167, 0.1168, 0.1169, 0.117, 0.1171,0.1172, 0.1173, 0.1174, 0.1175, 0.1176, 0.1177, 0.2177, 0.3177, 0.4177,0.5177, 0.6177, 0.7177, 0.8177, 0.9177, 1, 2, 3, 4, 5, and 10 weightpercent. For example, methanol can be present in an amount greater than0.0032 weight percent or in an amount greater than 0.1172 weight percentof the total composition of a crude PO stream from a PO/TBA process.

Acetaldehyde can be present in an amount within a range having a lowerlimit and/or an upper limit, each expressed as a weight percentage ofthe total composition of a crude PO stream from a PO/TBA process. Therange can include or exclude the lower limit and/or the upper limit. Theacetaldehyde lower limit and/or upper limit can be selected from 0,0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12,0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, 0.21, 0.22, 0.23, 0.24,0.25, 0.26, 0.27, 0.28, 0.29, 0.3, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36,0.37, 0.38, 0.39, 0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48,0.49, 0.5, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.6,0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.7, 0.71, 0.72,0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.8, 0.81, 0.82, 0.83, 0.84,0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96,0.97, 0.98, 0.99, 1, 2, 3, 4, 5, and 10 weight percent. For example,acetaldehyde can be present in an amount of greater than 0.03 weightpercent of the total composition of a crude PO stream from a PO/TBAprocess.

Water can be present in an amount within a range having a lower limitand/or an upper limit, each expressed as a weight percentage of thetotal composition of a crude PO stream from a PO/TBA process. The rangecan include or exclude the lower limit and/or the upper limit. The waterlower limit and/or upper limit can be selected from 0, 0.01, 0.02, 0.03,0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15,0.16, 0.17, 0.18, 0.19, 0.2, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27,0.28, 0.29, 0.3, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39,0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, 0.51,0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.6, 0.61, 0.62, 0.63,0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.7, 0.71, 0.72, 0.73, 0.74, 0.75,0.76, 0.77, 0.78, 0.79, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87,0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99,1, 2, 3, 4, 5, and 10 weight percent. For example, water can be presentin an amount of greater than 0.16 weight percent of the totalcomposition of a crude PO stream from a PO/TBA process.

Formaldehyde can be present in an amount within a range having a lowerlimit and/or an upper limit, each expressed as a weight percentage ofthe total composition of a crude PO stream from a PO/TBA process. Therange can include or exclude the lower limit and/or the upper limit. Theformaldehyde lower limit and/or upper limit can be selected from 0,0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01,0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13,0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, 0.21, 0.22, 0.23, 0.24, 0.25,0.26, 0.27, 0.28, 0.29, 0.3, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37,0.38, 0.39, 0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49,0.5, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.6, 0.61,0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.7, 0.71, 0.72, 0.73,0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85,0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97,0.98, 0.99, 1, 2, 3, 4, 5, and 10 weight percent. For example,formaldehyde can be present in an amount of greater than 0.005 weightpercent of the total composition of a crude PO stream from a PO/TBAprocess.

Tables 1 and 2 show exemplary concentrations of key impurities in acrude PO stream from a PO/TBA process, each expressed as a weightpercentage of the total composition of a crude PO stream from a PO/TBAprocess.

TABLE 1 Component Average weight percent MeF 0.06 Methanol 0.1172Acetaldehyde 0.03 Water 0.16 Formaldehyde 0.005

TABLE 2 Component Average weight percent MeF 0.06 Methanol 0.0032Acetaldehyde 0.03 Water 0.16 Formaldehyde 0.005

Without wishing to be bound by theory, a major problem is caused by thereaction of methanol with formaldehyde. As shown in Scheme 8, analdehyde, like formaldehyde, can react with an alcohol, like methanol toform a hemiacetal. According to Scheme 8, R1 and R2 can be hydrogen, ora C₁₋₁₀ alkyl.

Formation of an acetal can occur when the hydroxyl group of a hemiacetalbecomes protonated and is lost as water, as illustrated in Scheme 9,wherein R1, R2, and R3 can be hydrogen, or a C₁₋₁₀ alkyl.

Both formaldehyde and methanol would be lights by themselves, but theformation of hemiacetals and acetals can make them heavy. Subsequently,these addition products can travel downstream where temperaturesincrease and the reaction reverses. When the reaction reverses,aldehydes can become trapped with the desired propylene oxide product.

Referring to FIG. 1, an embodiment of the present disclosure relates toa separation system 4 for removing impurities from a crude PO stream 10from a PO/TBA process. The crude PO stream 10 can include, but is notlimited to, all of the impurities described above along with the desiredproduct, propylene oxide. The crude PO stream 10 can be fed into adistillation column, such as solvent-lights column 1. Most of theimpurities (for example, methanol) in crude PO stream 10 can be removedin an overhead stream 11 and sent to a cooler system 6 (see also FIG. 2)which may provide for partial condensation. The remaining vapor stream12 can be forwarded from the cooler system 6 to an overhead condensersystem 7 (see also FIG. 2) to give a vapor purge stream 71 and a liquidpurge stream 72, for example. All or some of the condensation exitingthe cooler system 6 may be sent as a wash inlet stream 13 to a waterwash apparatus 2, with a portion of the condensation optionally taken asreflux back to the solvent-lights column 1.

For instance, in the illustrated example of FIG. 1, a reflux stream 14can be taken from wash inlet stream 13 and recycled to thesolvent-lights column 1. Wash inlet stream 13 can be fed into the waterwash apparatus 2. A water inlet stream 20 can also be fed into the waterwash apparatus 2. Solvents recovered from the water wash apparatus 2 canbe recycled via recycle stream 21 to the solvent-light column 1. Anaqueous purge stream 22 can also be removed from the water washapparatus 2.

The solvent-lights bottom product stream 15 of solvent-lights column 1can be passed through a solvent-lights reboiler 5. A solvent-lightsreboiler vapor stream 16 can be fed back to the solvent-lights column 1.A solvent-lights reboiler bottoms stream 17 can be added to solventstripper column 3. An overhead product stream 34 of the solvent strippercolumn 3 can include the desired propylene oxide product. Overheadproduct stream 34 can be processed to achieve further separation ofpropylene oxide. A bottoms product stream 31 of the solvent strippercolumn 3 can be recycled to the water wash apparatus 2 via stream 33and/or to the solvent-light column 1 via stream 32.

An embodiment of the solvent-lights column 1 is now described in greaterdetail. The solvent-lights column 1 can be made of any suitablematerial, including but not limited to carbon steel or stainless steel.The solvent-light column 1 can include any suitable number of trays ortheoretical trays, for example, about 25 theoretical stages. In certainembodiments, crude PO stream 10 can be added at tray 11 to 15, countingfrom the bottom. A packing material can be employed in thesolvent-lights column to enhance vapor-liquid contact. Suitable packingmaterials can be made from any material including glass, metal, plastic,and ceramic. The packing can be structured or dumped. Trays such assieve trays, bubble cap trays or valve trays can also be used.

As described below, water wash apparatus 2 is effective in removing keylight impurities such as methyl formate, formaldehyde, acetaldehyde, andmethanol. This helps keep hemiacetal or acetal formation as low aspossible in the solvent-lights column 1. As already discussed,hemiacetal and acetal could enter into the solvent-light bottom productstream 15 and later breakdown in downstream columns as aldehydes tocontaminate the propylene oxide product.

Unexpected and beneficial results can be obtained by operatingsolvent-lights column 1 and/or solvent-lights reboiler 5 at atemperature within a range having a lower limit and/or an upper limit,each expressed in degrees Celsius. The range can include or exclude thelower limit and/or the upper limit. The reboiler temperature lower limitand/or upper limit can be selected from 50, 51, 52, 53, 54, 55, 56, 57,58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75,76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93,94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108,109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122,123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136,137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150,151, 152, 153, 154, 155, 156, 157, 158, 159, and 160 degrees Celsius.For example, the solvent-lights reboiler 5 can be operated at atemperature of 114 degrees Celsius or in a range of from 80 to 120degrees Celsius.

Additionally or alternatively, unexpectedly beneficial results can beobtained by operating solvent-lights column 1 at a pressure within arange having a lower limit and/or an upper limit, each expressed inpsig. The range can include or exclude the lower limit and/or the upperlimit. The pressure lower limit and/or upper limit can be selected from0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37,38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55,56, 57, 58, 59, and 60 psig. For example, the solvent-lights column 1can be operated at a pressure of 30 psig or in a range of from 20 to 50psig.

Without wishing to be bound by theory, it is believed that by operatingsolvent-lights reboiler 5 at temperatures and/or pressures in theabove-recited ranges, heavies such as hemiacetal or acetal formed insolvent-lights column 1, can break down into aldehydes. These aldehydescan then be removed to the overhead of the solvent-lights column 1 andeventually be purged out via water wash apparatus 2 or via the remainingvapor stream 12 instead of staying in the column bottom andcontaminating the PO product.

One embodiment of the present disclosure relates to a method forremoving impurities from a crude PO stream 10 from a PO/TBA process. Thecrude PO stream 10 can have a composition as previously defined. Themethod can include passing the crude PO stream 10 through a distillationcolumn, such as solvent-lights column 1. The distillation column can beoperated at the temperatures and pressures as previously defined.

Vapor Liquid Equilibrium (VLE) studies confirm that at increasedpressure or temperature, acetaldehyde relative volatility to POdecreases, which indicates a more difficult aldehyde separation in thesolvent-light column 1 at a higher pressure when alcohols are notpresent. Unexpectedly, with alcohols present, higher temperature andpressure result in a greater relative volatility of acetaldehyderelative to PO than at a lower pressure. Results of the experimental VLEstudies are given in Tables 3 and 4.

Table 3 presents the results of an experiment of binaryacetaldehyde-propylene oxide VLE. Data was obtained for three pressures,14.7 psia, 29.2 psia, and 60 psia. This binary VLE data set shows adeclining acetaldehyde to PO volatility at increasing pressure ortemperature. Since the mixtures do not contain methanol, the effect onvolatility could be only pressure or temperature although there is apossibility of acetaldehyde dimer or trimer formation. However, theacetaldehyde dimer or trimer formation equilibrium would be similar tohemiacetal/acetal equilibriums; they would be favored at lowpressure/temperature. Therefore, the effect of pressure/temperatureobserved here could be slightly reduced. This set of data was obtainedat starting acetaldehyde concentration of 5300 ppm.

TABLE 3 Relative Volatility of Acetaldehyde in Crude Propylene Oxidewithout methanol¹ Composition α Pressure Temperature (weight percent) K(AA/ (psia) (° C.) Component Vapor² Liquid² values PO) 14.7 32 AA 0.7520.421 1.786 1.791 PO 99.248 99.579 0.997 29.2 55.7 AA 0.717 0.461 1.5561.560 PO 99.283 99.529 0.994 60.0 79.8 AA 0.649 0.418 1.554 1.557 PO99.351 99.582 0.998 Note: ¹Contains 0.53% Acetaldehyde ²Normalized

Unexpected and beneficial results can also be obtained by reducing theamount of water, methanol, and/or glycol concentration in thesolvent-light column 1. With reduced methanol (MeOH) in the crude POstream 10, both formaldehyde and acetaldehyde removal can be improved,as indicated by the reduced aldehyde level in overhead product stream 34from the solvent stripper column 3. VLE (Table 4) showed thatacetaldehyde relative volatility to PO declines with increased methanolconcentration.

Table 4 presents VLE data for PO-acetaldehyde-methanol system, for theeffect of methanol on acetaldehyde volatility in propylene oxide. Theresults demonstrate that at atmospheric pressure or low temperature,acetaldehyde volatility to PO declines with increasing methanolconcentration in PO. As methanol concentration reaches about 2.5-3 wt %,acetaldehyde volatility to PO is approaching 1 which makes acetaldehydeinseparable from PO. When methanol concentration increases to about 4 wt%, acetaldehyde become heavier than PO with a relative volatility to POnear 0.82. This phenomenon is believed to be caused by the formation ofhemiacetal and acetal at increased methanol concentration even thoughacetaldehyde concentration was low at only around 50 ppm. Additional VLEdata were obtained at about 3 wt % methanol and elevated pressure orincreased temperature. By comparing data obtained at atmosphericpressure, 16 psig and 28.7 psig, the results show that acetaldehydevolatility to PO increases with increasing pressure or temperature whenmethanol is present at a same methanol concentration. The equilibriumformation of hemiacetal/acetal becomes less favored at elevatedtemperatures. Thus, it is desirable to remove methanol first so thataldehydes will distill overhead in the solvent-lights column 1. Ifaldehydes are not completely removed, it is desirable to increase thepressure of the solvent-lights column 1 to break the hemi-acetals, sothat the aldehydes can be taken overhead.

TABLE 4 VLE of Synthetic PO-AA-MeOH Mixtures at Atmospheric PressureComposition Run T P (by weight) K α # (° C.) (mmHg) Component VaporLiquid values (AA/PO)  1 33.3 755.8 AA 96 ppm 56 ppm 1.74 1.74 MeOH —  5ppm — PO 99.9904%  99.9939% 1.00  2 33.0 754.3 AA 99 ppm 57 ppm 1.791.76 MeOH 582 ppm  666 ppm  0.87 PO 99.93199%  99.9278% 1.00  3 33.4748.4 AA 85 ppm 53 ppm 1.61 1.61 MeOH 0.3772%  0.4984% 0.76 PO 99.6143% 99.4963% 1.00  4 32.8 747.5 AA 83 ppm 51 ppm 1.62 1.62 MeOH 0.8165% 1.0476% 0.78 PO 99.1752%  98.9493% 1.00  5 32.4 754.3 AA 68 ppm 51 ppm1.35 1.33 MeOH 2.3812%  3.4437% 0.69 PO 97.612% 96.5512% 1.01  6* 34.7750.9 AA 56 ppm 52 ppm 1.09 1.08 MeOH 2.6061%   3.50% 0.74 PO 97.3883% 96.4856% 1.01  7 32.7 755.1 AA 44 ppm 52 ppm 0.86 0.84 MeOH 3.7000% 5.8658% 0.63 PO 96.2956%  94.1290% 1.02  8 33.5 746.9 AA 44 ppm 52 ppm0.85 0.82 MeOH 4.2013%  7.1129% 0.59 PO 95.7943%  92.8819% 1.03  9* 34.7750.9 AA 56 ppm 52 ppm 1.09 1.08 MeOH 2.6061%  3.5092% 0.74 PO 97.3883% 96.4856% 1.01  10* 56.4  16 psig AA 63 ppm 48 ppm 1.33 1.32 MeOH 2.9799% 3.3628% 0.89 PO 97.0138%  96.6325% 1.00  11* 68.1 28.7 psig AA 67 ppm47 ppm 1.42 1.42 MeOH 3.2594%  3.3560% 0.97 PO 96.7339%  96.6393% 1.00*Run # 6 was conducted in a steel recirculation still. *Runs # 9-11 wereconducted in a stainless-steel still.

The water wash apparatus 2 will is now described in greater detail. Thewash inlet stream 13 from the solvent-lights column 1 can be sent towater wash apparatus 2. The water wash in water wash apparatus 2 can becarried out by mixing the wash inlet stream 13 (having propylene oxideand impurities) with water and solvent. In particular, water suppliedvia water inlet stream 20 can be used to remove the impurities frompropylene oxide. A solvent (from stream 33) can be used to reducepropylene oxide loss into the water phase. Adequate mixing is beneficialto accomplish preferable impurity removal. Adequate coalescing, andenough residence time in the water wash apparatus 2 is also beneficialto reduce entrainment of the aqueous phase in the organic effluent. Theorganic effluent can be recycled back to the solvent-lights column 1 viarecycle stream 21. An aqueous purge stream 22 with a high concentrationof impurities can be purged from the water wash apparatus 2.

The organic effluent in recycle stream 21 can include an amount ofaqueous phase within a range having a lower limit and/or an upper limit,each expressed as weight percentages. The range can include or excludethe lower limit and/or the upper limit. The lower limit and/or upperlimit for the amount of the aqueous phase in the organic effluent of thewash can be selected from 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.2,0.3, 0.4, 0.5, 0.7, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10, weight percent.For example, less than 0.1% of the aqueous phase can be present in theorganic effluent of the wash or 10% of the aqueous phase can be presentin the organic effluent of the wash.

Key light impurities to remove are methyl formate, formaldehyde,acetaldehyde, and methanol. Most of these impurities in thesolvent-lights column overhead stream 11 (an example of an impuritystream) can be removed through a combination of remaining vapor stream12 and aqueous purge stream 22 from water wash apparatus 2. Lab waterwash tests have demonstrated the effective removal of these key lightimpurities.

The solvent stripper column 3 is now described in greater detail. Thesolvent stripper column 3 can be made of any suitable material,including but not limited to stainless steel or carbon steel. Thesolvent stripper column 3 can include any suitable number of trays ortheoretical trays, for example, about 10 trays. Solvent-lights reboilerbottoms stream 17 can be added at tray 1-10, for example at tray 5. Apacking material can be employed in the solvent stripper column 3 toenhance vapor-liquid contact. Suitable packing materials can be madefrom any material including glass, metal, plastic, and ceramic. Ifpacking is used, it can be structured or dumped, and the like. If traysare used, then can be sieve trays, bubble cap trays or valve trays, andso on.

Referring to FIG. 2, more exemplary detail of the solvent-lights column1, the cooler system 6, and overhead condenser system 7 is shown. Inoperation, the overhead stream 11 from the solvent-light column 1 can bepassed into a cooler 61, which utilizes cooling fluid introduced viacooling inlet line 64 and removed via cooling outlet line 63. Thepartially condensed outlet stream 65 from the cooler 61 flows into areflux drum 62. Liquid from the reflux drum 62 may be split into thereflux stream 14 and the wash inlet stream 13 mentioned above withrespect to FIG. 1. The remaining vapor stream 12 from the reflux drum 62can be fed to a vapor condenser 73, supplied with cooling glycol (orother refrigerant or cooling medium) which enters the vapor condenser 73via refrigerant inlet 76 and exits via refrigerant outlet 77. Thecondenser outlet 75 can be fed into a separator 74 to give the vaporpurge stream 71 and the liquid purge stream 72 mentioned above withrespect to FIG. 1.

Referring to FIG. 3, the solvent stripper column 3, according to oneembodiment as used in a pilot plant, is depicted in greater detail. Notethat the specific dimensions referenced below refer to one particularembodiment and are not meant to limit the scope of the claimedinvention. The solvent stripper column 3 was made from 3″ Schedule 40pipe. The entire height including the solvent stripper column reboiler83 was 88 inches tall. The solvent stripper column 3 included a firstpacked section 81 and a second packed section 82, each packed sectionwas 28¾ inches tall with 24 inches of packing. The packing was made of0.24″ Pro-pak™ packing, supported by conical screens resting on ringswelded to the internal diameter of the column. Distribution rings werealso used at the top of each packed section to ensure even distributionof liquid from above, over the packing.

Still referring to FIG. 3, the feed point 80 was in the middle of thevertical height of solvent stripper column 3, between the first packedsection 81 and the second packed section 82. A feed, depicted assolvent-lights reboiler bottoms stream 17 in FIG. 1, was added to thesolvent stripper column 3 at the feed point 80. The solvent strippercolumn 3 was operated at 4 to 5 psig. The steam flow to the solventstripper column reboiler 83 at the base of the stripper column 3 wascontrolled to hold the weight percentage of PO in the bottoms at 0.5 to1.5 wt %. Vapor was removed from the top of the stripper column 3, andfed to a total condenser (not shown in FIG. 3). The condensed liquid wassplit into two parts. One part was fed back to the top of the solventstripper column 3 as reflux. The rest of the liquid distillate was takenas the overhead product stream 34 shown on FIG. 1.

EXAMPLES

The following examples were carried out in a continuous pilot plant. Theoverview of the pilot unit is shown in FIG. 1. Additional details of asolvent-lights column 1, used in the examples are shown in FIG. 2.Additional details of solvent stripper column 3 are shown in FIG. 3. Thesolvent-lights column 1 employed in the examples had a 2″ insidediameter and contained a bed of Pro-pak™ stainless steel protrudedpacking that was 11 feet deep. The Pro-pak™ stainless steel protrudedpacking was 0.24″ size. The solvent stripper column 3 in FIG. 1 is alsoshown in more detail on FIG. 3. The solvent stripper column 3 was 3″inside diameter and contained a bed of Pro-pak™ stainless steelprotruded packing, 0.24″ size, which was 4 feet deep.

Example 1

Example 1 describes the test period when the pilot unit solvent-lightscolumn 1 as shown in FIGS. 1 and 2 was operated first at 25 psig. Thecrude PO stream 10 comprising crude propylene oxide (an intermediatestream from a PO/TBA process) was fed to a point on the solvent-lightscolumn 1 at the middle of the column. Table 5 shows the concentrationsof key impurities in the feed stream, each expressed as a weightpercentage of the total composition.

TABLE 5 Component Average weight percent MeF 0.06 Methanol 0.1172Acetaldehyde 0.03 Water 0.16 Formaldehyde 0.005

A plurality of temperature probes extending into the solvents lightscolumn 1 were positioned along the vertical length of the solvent-lightscolumn 1.

The temperature of the crude PO stream 10 was 27 degrees Celsius and theflow rate was 3.0 kg/hr. Stream 32, having a lean solvent, pumped fromthe bottom of the solvent stripper column 3, was introduced at the topof the solvent-lights column 1 as shown in FIG. 1. (The solvent strippercolumn 3 is also shown in greater detail in FIG. 3.) The flow rate oflean solvent in stream 32 was 21.5 kg/hr. Reflux stream 14 wasintroduced into solvent-lights column 1 at a rate of 1.5 kg/hr.

Wash inlet stream 13 was introduced into water wash apparatus 2 at arate of 185 gm/hr. Two other streams were fed to the water washapparatus 2: deionized water at a rate of 100 gm/hr and lean solventfrom the bottom of the solvent stripper at a rate of 2.4 kg/hr. Thewater wash apparatus 2 consisted of three parts: a mixer, a coalescerand a decanter. The mixer was a 4-inch section of 1/16″ OD tubing havingan inside diameter of 0.030″. Downstream of the mixer was a coalescer(not illustrated) which was a 1-foot long bed of glass wool in a ⅜″ ODtube. Downstream of the coalescer was a decanter (not illustrated) wherethe organic and aqueous phases were separated. The decanter was avertical glass pipe, 2.0″ ID by 12″ tall. The washed organic phaseoverflowed from the top of the decanter and was sent to the top of thesolvent-lights column 1. The aqueous bottom layer from the decanter,rich in methanol, methyl formate, acetaldehyde and formaldehyde, wassampled and collected. The organic and aqueous products from thedecanter were used to calculate partition coefficients for the keyimpurities, as shown in Table 6. Partition Coefficient for eachcomponent (i) was calculated based on the following definition:

${{Partition}\mspace{14mu}{Coefficient}} = \frac{{Weight}\mspace{14mu}{fraction}\mspace{14mu}{in}\mspace{20mu}{Aqueous}\mspace{14mu}{phase}}{{Weight}\mspace{14mu}{fraction}\mspace{14mu}{in}\mspace{14mu}{Organic}\mspace{14mu}{phase}}$

TABLE 6 Component Average Partition Coefficient Methyl Formate 1.6Methanol 57 Acetaldehyde 6.6 PO 0.8 Formaldehyde 190

Table 6 shows that methanol, acetaldehyde and formaldehyde are easilyextracted by the water wash block, since the partition coefficients arehigh.

Table 7 provides exemplary temperature, pressure and flow rate data forthe pilot unit operation.

TABLE 7 Stream Temperature Pressure Flow Rate 10 69-84° C. 25-30 psig2.7-3.3 kg/hr 11 77-84° C. 25-30 psig 1.65-1.72 kg/hr 12 63-72° C. 25-30psig  2-13 gm/hr 13 50-68° C. 25-30 psig 160-200 gm/hr 14 50-68° C.25-30 psig 1.49-1.5 kg/hr 15 106-119° C.  25-30 psig 24-28 kg/hr 16106-119° C.  25-30 psig 17 16-20° C. 25-30 psig 24-28 kg/hr 20 20-26° C.25-30 psig 100-101 gm/hr 21 43-46° C. 25-30 psig 2.3-2.8 kg/hr 22 43-46°C. 25-30 psig 102-118 gm/hr 31 20-26° C. 25-30 psig 22.2-25.6 kg/hr 3220-26° C. 25-30 psig 20-23 kg/hr 33 20-26° C. 25-30 psig 2.2-2.6 kg/hr34 69-84° C.  3-4 psig 2.7-3.3 kg/hr

The vapors from solvent-lights column 1, which did not condense incooler 61 shown in FIG. 2 were collected and analyzed. Thesolvent-lights reboiler bottoms stream 17 from the solvent-lights column1 was sent to the middle of the solvent stripper column 3, as shownFIG. 1. The solvent stripper column 3 was operated at 4 psig. A purposeof the solvent stripper column 3 was to recover the propylene oxideproduct as a distillate (overhead product stream 34) and the leansolvent as the bottoms product stream 31. The feed rate to the solventstripper column 3 was 26.9 kg/hr. The reflux rate to the solventstripper column 3 was 8.0 kg/hr. As mentioned earlier, the bottomsproduct stream 31 from the solvent stripper column 3 was split into twostreams (via stream 32 and stream 33), with stream 32 feeding the top ofthe solvent-lights column 1 and stream 33 feeding the water washapparatus 2 on FIG. 1.

As the pressure of the solvent-lights column 1 was increased from 25psig to 30 psig, the operating temperatures at the solvent-light column1 also increased by about 5 degrees Celsius. At higher columntemperature, a large amount of hemiacetals and/or acetals are convertedto the form of aldehyde plus alcohol. Aldehyde and alcohol are thendistilled overhead in the solvent-lights column 1 and removed by bothwater wash and vapor purge.

Formaldehyde is primarily removed into aqueous purge. Acetaldehyde isremoved into both purges. As shown in Table 6 water wash operation,formaldehyde is favorably partitioning into the aqueous phase.

As shown in Table 8, with higher temperatures at the solvent-lightscolumn 1, formaldehyde in the final pilot plant product (contained inoverhead product stream 34 from the solvent stripper column 3) isreduced from 25.4 ppm to 7.8 ppm and acetaldehyde is reduced from 6.4ppm to 4.8 ppm. This was an unexpected and extremely beneficial result.

TABLE 8 Impact of Higher Distillation Pressure and Temperature onAldehyde Removal Solvent-Lights Column 1 Solvent- light Solvent-LightsColumn 1 Solvent- bottom Overhead (wash inlet Solvent Stripper OverheadLight product stream13) Product Stream 34 Overhead 15 Avg. Avg. AverageAverage Pressure 11 Temp Temp Formaldehyde, Acetaldehyde, FormaldehydeAcetaldehyde (psig) (° C.) (° C.) (wt. %) (wt. %) (ppm) (ppm) 25 77.178.5 0.0422 1.222 25.4 6.4 30 82.1 83.1 0.0683 1.266 7.8 4.8

Example 2

Unexpectedly beneficial results can also be obtained by reducing theamount of water, methanol, and/or glycol concentration in thesolvent-lights column 1 feed. Two methanol (MeOH) concentrations weretested using the same pilot unit as describe in Example 1. One test useda propylene oxide feed containing 0.1172 wt % MeOH, as shown in Table 5.The other test used a feed having 0.0032 wt % of MeOH, as shown in Table9. The feed stream comprising a propylene oxide feed stream was a crudePO stream from a PO/TBA process. Both Table 5 and Table 9 show theconcentrations of key impurities in the feed stream, each expressed as aweight percentage of the total composition of a crude PO stream from aPO/TBA process.

TABLE 9 Component Average weight percent MeF 0.06 Methanol 0.0032Acetaldehyde 0.03 Water 0.16 Formaldehyde 0.005

With reduced MeOH in PO feed, both formaldehyde and acetaldehyde removalwas unexpectedly improved, as indicated by the reduced aldehyde level insolvent stripper overhead product stream 34. Without wishing to be boundby theory, it is possible that the improvement is due to both enhancedaldehyde-propylene oxide vapor liquid equilibrium (VLE) and lesscarryover of hemiacetals or acetals into the solvent stripper column 3from the solvent-light column 1. Table 10 summarizes the resultsobtained.

TABLE 10 Impact of Methanol Concentration on Aldehyde Removal OverheadProduct Solvent Light Column Overhead Stream 34 Crude PO (wash inletstream13) Average Feed 10 Average Average Average Acetalde- wt %Formaldehyde Acetaldehyde Formaldehyde hyde MeOH (wt %) (wt %) (ppm)(ppm) 0.1172 0.0683 1.266 7.8 4.8 0.0032 0.0736 1.275 3.6 3.5

FIG. 4 is an overview of an exemplary propylene oxide (PO) separationsystem 100 for removing impurities from a crude propylene oxide (PO)stream 102 from a PO/TBA reactor process. The crude PO stream 102 may bean effluent stream from a reactor of a PO/TBA process, for example, andmay include impurities described above along with the desired product,PO.

In certain examples, the crude PO stream 102 is not subjected toupstream removal of heavy components such as in a heavies column priorto being fed to the PO separation system 100. Thus, the crude PO stream102 entering the PO separation system 100 may have a significant amountof water and methanol, for instance. Examples of impurities given inexemplary weight percentage of the crude PO stream 102 are listed inTable 11. Of course, other weight percentages for these impurities areaccommodated by the present techniques.

TABLE 11 Component Average weight percent MeF 0.06 Methanol 0.34Acetaldehyde 0.03 Water 0.47 Formaldehyde 0.0047

In embodiments, the PO separation system 100 includes a front-end 104and a back-end 106. In general, the front-end 104 removes lightimpurities, water, and water-soluble impurities (e.g., methanol) fromthe crude PO stream 102, as well as some solvent, and discharges a POstream 108 having PO, hydrocarbon solvent, and some impurities to theback-end 106. In certain embodiments, the level of impurities in the POstream 108 is relatively low and predominant components may be PO andsolvent. The back-end 106 generally removes the hydrocarbon solvent (andimpurities) from the PO stream 108 to give PO product stream 109.

Hydrocarbon solvent may be added (not shown) to the front-end 104 tofacilitate formation of aqueous (water) and organic (solvent) phases inthe front-end 104. The PO typically has an affinity for the organic(solvent) phases/streams in the front-end 104. Again, the back-end 106removes the hydrocarbon solvent from the PO stream 108 and discharges aPO product stream 109. The source of the hydrocarbon solvent to thefront-end 104 may be solvent recycled from the back-end 106 and/or freshsolvent.

As discussed in detail below, the front-end 104 of the separation system100 may include a distillation column, such as a solvent-lights column,and a solvent stripper column, and the like. Again, the front-end 104removes light impurities and aqueous impurities from the crude PO stream102, and forwards a PO stream 108 to the back-end 106. The PO stream 108may be further processed in a back-end 106 of the separation system 100which may include an extraction column, solvent column, and PO productcolumn, and so forth. The back-end 106 subjects the crude PO to solventextraction and also removes light and heavy impurities to give POproduct stream 109.

As also discussed below, to facilitate removal in the front-end 104 ofat least the water and methanol impurities from the crude PO stream 102and from the solvent-lights column, the techniques may beneficiallyprovide for a combination (FIG. 5) of a decanter and water wash on thesolvent stripper column overhead, and/or a side draw (FIG. 7) from thesolvent-lights column. Moreover, in general, the present techniques mayadvantageously provide for a grassroots facility or for retrofit ofexisting equipment and operations.

FIG. 5 is an exemplary front-end 104A of the propylene oxide separationsystem 100. The crude PO stream 102 (e.g., a PO reactor effluent streamof a PO/TBA process) is fed into a distillation column, such assolvent-lights column 110. A majority of the PO and hydrocarbon solventin the incoming crude PO stream 102 discharges in a product stream(bottoms stream 140) from the bottom of the solvent-lights column 110.(As noted below, bottoms stream 140 is the portion of the bottomsdischarge from the solvent-lights column 110 not recycled to thesolvent-lights column 110 through the solvent-lights reboiler 143.) Mostof the impurities such as light and aqueous impurities in the crude POstream 102 are removed in an overhead stream 112 and sent to an overheadcondenser 114 (e.g., shell and tube heat exchanger). The overheadcondenser 114 may provide for partial condensation of the overheadstream 112 in certain examples. A cooling medium (e.g., cooling towerwater) is fed to the utility side of the overhead condenser 114 inembodiments.

Components in the overhead stream 112 entering the overhead condenser114 that are not condensed can be purged from the system 100 (orfront-end 104A) via a vapor stream 116 purge. These non-condensedcomponents in vapor stream 116 may be sent to another process,discharged as waste, and the like. If desired, the non-condensedcomponents in vapor stream 116 may be subjected to further localprocessing, such as in an additional condenser operating at lowertemperature than the overhead condenser 114, and so forth. Thenon-condensed components in vapor stream 116 may include acetaldehyde,methyl formate, and other undesired impurities.

A condensed overhead stream 118 discharges from the process side of theoverhead condenser 114 and is sent to a decanter 120, which providesresidence time for separation of an organic phase and an aqueous phase(not shown in the figures). In one example, the amount of water andmethanol in condensed overhead stream 118 is 4 weight % water and 3weight % methanol. An organic stream 122 from the organic phase indecanter 120 may be sent as reflux to the solvent-lights column 110. Anaqueous stream 124 from the aqueous phase in decanter 120, which has themajority of the methanol and water in the portion of the condensedoverhead stream 118 entering decanter 120, may be sent from decanter 120to a water wash system 126 in this example.

Thus, the decanter 120 may facilitate removal of relatively largeamounts of water and methanol from the condensed overhead stream 118 sothat beneficially less water and less methanol are refluxed back to thesolvent-lights column 110. Therefore, advantageously, lower amounts ofmethanol and water accumulate in the solvent-lights column 110. Use ofthe organic stream 122 as the relatively dry reflux reduces theprobability of separate water phase formation in the solvent-lightscolumn 110.

Solvent 128 (discussed below), which may be a hydrocarbon (e.g. C8-C10),may be added to the solvent-lights column 110, to the decanter 120,and/or to the water wash system 126. Addition of solvent 128 to thedecanter 120 may facilitate the formation and separation in the decanter120 of the aqueous phase having the undesired methanol and water. Asindicated, an aqueous stream 124 is sent from the decanter 120 to thewater wash system 126 to discharge impurities such as methanol, water,methyl formate, acetaldehyde, glycols, and the like, from the system 100(or front-end 104A) via the downstream aqueous purge 130 of the waterwash system 126.

Water 132 (e.g., tap water, treated water, demineralized water, etc.) isadded to the water wash system 126 to drive the downstream aqueous purge130 of impurities from the system 100. The water wash system 126 mayhave a vessel or coalescer (not shown), for example, to provide volumefor the water wash. The water wash system 126 may also include anupstream mixer, for example a static mixer (also not shown) to providefor mixing of the aqueous stream 124 and the solvent 128 prior to entryto the vessel or coalescer of the wash system 126. Of course, otherconfigurations for the water wash system 126 may be accommodated.

A source of glycol impurities may be various solvents in the system 100that deteriorate over time in the presence of water and methanol, forinstance, to form glycols. An advantage of removing the impurities (forexample, water and methanol) is that the hydrocarbon solvents present inthe system 100 may deteriorate less.

A wash organic stream 134 is sent from the water wash system 126 to thedecanter 120 for eventual reflux to the solvent-lights column 110 (viaorganic stream 122). Further, optionally, a portion of the condensedoverhead stream 118 from the overhead condenser 114 may bypass thedecanter 120 and be sent directly to the water wash system 126. In theembodiment of FIG. 5, a portion of the condensed overhead stream 118 issent to the decanter 120 and a portion bypasses the decanter 120 for thewater wash system 126.

The present techniques provide unique embodiments of the overheadconfiguration of the solvent-lights column 110 to remove lightimpurities via a vapor purge of non-condensed components (vapor stream116) and via a downstream aqueous purge 130 from the water wash system126. The decanter 120 provides volume and residence time, and a unitoperational to receive solvent addition to allow formation of theaqueous phase (giving aqueous stream 124) having significant amounts ofwater, methanol, and other aqueous-phase impurities.

Advantageously, removal of these light impurities such as methylformate, formaldehyde, acetaldehyde, and methanol via the downstreamaqueous purge 130 (and thus reducing the amount of such impurities inthe reflux to solvent-lights column 110) reduces hemiacetal or acetalformation in the solvent-lights column 110. Such heavier-formedcomponents have lower boiling points and could undesirably discharge inthe product stream (bottoms stream 140) from solvent-lights column 110.Further, these hemiacetal or acetal compounds could later breakdown indownstream columns into aldehydes and contaminate the PO product.

As indicated, present embodiments of the solvent-lights column 110 andits overhead configuration reduce hemiacetal or acetal formation in thesolvent-lights column 110. Moreover, the disclosed techniques facilitatecapability for the front-end 104A of the separation system 100 (FIG. 4)to receive a crude PO stream having relatively high amounts of water andmethanol, for example, directly to the solvent-lights column 110.

The aforementioned product stream from the bottom of the solvent-lightscolumn 110 is labeled as bottoms stream 140 in FIG. 5. This bottomsstream 140, having a majority of the PO entering the column 110, can besent to a solvent stripper 142. As is typical with distillation columns,some of the bottom discharge from the column 110 may be vaporized in asolvent-lights reboiler 143 and returned as vapor to the solvent-lightscolumn 110. Steam or steam condensate, for example, may be fed to theutility side of the solvent-lights reboiler 143. Bottoms stream 140 isthe portion of the bottoms discharge from the solvent-lights column 110not recycled to the solvent-lights column 110 through the solvent-lightsreboiler 143. The bottoms stream 140 is processed in the solventstripper 142 to remove solvent from the PO product in the bottoms stream140.

At the solvent stripper 142, solvent is removed via a bottoms discharge.A portion of the bottoms discharge may be sent through a solventstripper reboiler 146 and returned as vapor to the solvent stripper 142.Steam or steam condensate may be fed as the heating medium, for example,to the utility side of the solvent stripper reboiler 146. The remainingbottoms discharge is the solvent stripper bottoms stream 144, which maybe combined in this embodiment with fresh solvent or with a solventrecycle such as recycle solvent 149 from the exemplary back end 106A(FIG. 6) of the separation system 100, and so on, to result in theaforementioned solvent 128 fed to the solvent-lights column 110,decanter 120, and/or water wash system 126. The recycle solvent 149 maybe from the bottoms stream 148 of a solvent column 162, for example, inthe exemplary back-end 106A (FIG. 6).

A majority of the PO received at the solvent stripper 142 via bottomsstream 140 discharges in a solvent stripper overhead stream 150. Thissolvent stripper overhead stream 150 may be condensed in stripperoverhead condenser 152. The cooling medium fed to the utility side ofthe stripper overhead condenser 152 may be cooling tower water or othercooling fluid. A portion of the condensed solvent stripper overheadstream 150 exiting the condenser 152 may return to the solvent stripper142 as reflux. The remaining portion of the condensed solvent stripperoverhead stream 150 exiting the condenser 152 may be forwarded as adistillate (PO stream 108A in this example) to the exemplary back-end106A (see FIG. 6) of the separation system 100 (FIG. 4) for furtherprocessing to remove impurities from the PO in PO stream 108A. The POstream 108A sent to the exemplary back-end 106A may be analogous to POstream 108 of FIG. 4.

Lastly, the exemplary equipment contemplated in the exemplary front end104A of the separation system 100 may be commercial scale. Therespective diameters and heights of the solvent-lights column 110 andthe solvent stripper 142 may be sized as a function of the design basisfor the mass flow rate and composition of the incoming crude PO stream102, for instance. Further, in one example, the number of theoreticalstages in the solvent-lights column 110 is about 25, and the crude POstream 102 is fed into the solvents-lights column 110 at about stage 11to 15. Of course, other total numbers of theoretical stages, and feedpoints, are contemplated.

To provide for the theoretical stages, trays or packing may be employed,though trays may be typical. Trays may include sieve trays, bubble captrays or valve trays, and the like. The packing, which may be structuredor dumped, can be glass, metal, plastic, and ceramic, and so on. Themetallurgy or materials of construction of the various equipment in theexemplary front-end 104A, including the solvent-lights column 110 andthe solvent stripper 142, may be carbon steel, stainless steel,fiberglass reinforced polymer (FRP), nickel alloys, and so on. Suchmetallurgy or materials of construction may also be applicable to thecolumns and other equipment in the exemplary back-end 106A depicted inFIG. 6.

FIG. 6 is an exemplary back-end 106A associated with the exemplaryfront-end 104A (FIG. 5) of the separation system 100. The exemplaryback-end 106A includes an extraction column 160, solvent column 162, andPO column 164. For the sake of clarity, the respective reboiler andoverhead condenser (including any reflux system) for each column 160,162, and 164 are not shown.

The extraction column 160 receives as feed the portion of the condensedsolvent stripper overhead stream 150 from the solvent stripper 142 (FIG.5) collected as distillate as PO stream 108A. PO stream 108A issubjected to extraction with a solvent (e.g., C8-C10 hydrocarbon) inextraction column 160. The solvent used for extraction may come from thesolvent bottoms stream 148 of the downstream solvent column 162. Aproduct stream (extraction overhead stream 168), having the majority ofPO entering the extraction column 160, discharges overhead from theextraction column 160. An extraction bottoms stream 170, having solventand impurities discharges, from the bottom of the extraction column 160.

The extraction overhead stream 168 is condensed and sent to the POcolumn 164 where an overhead lights purge 172 is removed, a bottomsheavies purge 174 is removed, and a PO product stream 109A is dischargedas a product side draw. This PO product stream 109A may be analogous toPO product stream 109 of FIG. 4.

The extraction bottoms stream 170 from the extraction column 160 is fedto the solvent column 162 where a hydrocarbon purge 178 (e.g., C6) isremoved overhead and a solvent bottoms stream 148 (e.g., C8-C10) isremoved via a bottom discharge. As indicated, all or a portion of thissolvent bottoms stream 148 may be fed to the extraction column 160.Also, a take-off portion (recycle solvent 149) of the solvent bottomsstream 148 may be sent to unit operations in the front-end 104A (FIG.5).

A separation system 100 (FIG. 4) having the front-end 104A (FIG. 5) andback-end 106A (FIG. 6) may give a PO product stream 109A havingacceptable levels of impurities (i.e., within typical specifications)and at acceptable PO losses (e.g., less than 2%) in the separationsystem 100. Exemplary configurations of the front-end 104A giveacceptable and relatively low amounts of impurities in the solventstripper overhead stream 150 (FIG. 5) discharging from the solventstripper 142. The part per million (ppm) of certain impurities in thesolvent stripper overhead stream 150 are given in Table 12 for oneexample.

TABLE 12 Component Ppm MeF  <5 Acetaldehyde ~10 Methanol 5-10 Water <50

FIG. 7 is another example of a front-end 104B of the separation system100. The crude PO stream 102 is fed to a solvent-lights column system190. Exemplary details of the solvent-lights column system 190 are givenin FIGS. 8 and 9. The solvent-lights column system 190 dischargesimpurities received from the crude PO stream 102 via a vapor stream 116purge and an aqueous stream 124. Such impurities may include methanol,water, methyl formate, acetaldehyde, glycols, and the like. The vaporstream 116 purge may be sent to another process or discharged as waste,and so on. The aqueous stream 124 may be from an aqueous phase in adecanter in the solvent-lights column system 190, for example.

The aqueous stream 124 is sent to a water wash system 126. Varioussolvent-containing streams (e.g., back-end solvent 151 and distillate199, see below) from the back-end 106B (FIG. 10) and water 132 may becombined with aqueous stream 124 and routed through a mixer 204, forexample static mixer, prior to entering the water wash system 126. Anexample of a solvent stream from the back-end 106B (FIG. 10) added tothe aqueous stream 124 may be a back-end solvent 151 from the solventbottoms stream 148 of a solvent column 162, and so forth. Other streamsmay be added to the aqueous stream 124 such as overhead distillate 199from an extraction column 160 to purge formaldehyde from the back-end106B, for instance.

At the water wash system 126, the aforementioned impurities of methanol,water, methyl formate, acetaldehyde, glycols, and the like, aredischarged via a downstream aqueous purge 130. A wash organic stream 134may be sent from the water wash system 126 to the solvent-lights columnsystem 190. The water wash system 126 may include a vessel or coalescer,and/or other equipment.

The solvent-lights column system 190 discharges a product stream(bottoms stream 140) having a majority of the PO entering thesolvent-lights column system 190 in crude PO stream 102. The productstream may be a bottoms stream 140 from a solvent-lights column 110 (adistillation column) in the solvent-lights column system 190 (such asshown in subsequent FIGS. 8 and 9) or the solvent-lights column 110shown in FIG. 5. The product stream (e.g., bottoms stream 140) is sentto a solvent stripper 142, which may function similarly as discussedabove with respect to the front-end 104A (FIG. 5). At the solventstripper 142, solvent is removed via the solvent stripper bottoms stream144.

The solvent stripper bottoms stream 144 may be sent to thesolvent-lights column system 190. Optionally, additional solvent, suchas from solvent bottoms stream 148 of the solvent column 162 in theback-end 106B (FIG. 10), may be combined with the solvent stripperbottoms stream 144 to give the solvent 128 in route to thesolvent-lights column system 190. Thus, solvent 128 fed to thesolvent-lights column system 190 may be the solvent stripper bottomsstream 144 or a combination of the solvent stripper bottoms stream 144and the recycle solvent 149 from the back-106B (FIG. 10).

A majority of the PO received at the solvent stripper 142 (from bottomsstream 140) discharges in a solvent stripper overhead stream 150. Aportion of the condensed overhead stream is forwarded as distillate asPO stream 108B to the back-end 106B (see FIG. 10) of the separationsystem 100 for further processing to remove impurities from the PO.However, the amount of impurities in the overhead stream 150 and the POstream 108B is generally relatively low. This stream 108B sent to theback-end 106B may be analogous to the PO stream 108 of FIG. 4.

A beneficial aspect of the solvent-lights column system 190 is theformation and discharge of the aqueous stream 124 having theaforementioned impurities, and which may be accomplished in a variety ofconfigurations. FIGS. 8 and 9 provide respective examples of thesolvent-lights column system 190 having the solvent-lights column 110that gives aqueous stream 124 or a similar stream.

FIG. 8 is an exemplary solvent-lights column system 190-1 having thesolvent-lights column 110 that receives the crude PO stream 102, whichmay be received at various distillation stages along the solvent-lightscolumn 110. A solvent 128 is also fed to the column 110. In certainexamples, it may be beneficial to introduce the solvent 128 at or abovethe liquid side draw 222. An exemplary introduction point for thesolvent 128 is at stage or tray 3, for instance.

A decanter 120 is positioned as a side decanter to facilitate formationand discharge of the aqueous stream 124. A liquid side draw 222 from thesolvent-lights column 110 having some PO and also having water,methanol, acetaldehyde, and other impurities from the solvent-lightscolumn 110 is fed to the decanter 120. A purpose of the decanter 120 maybe to facilitate removal of water and other aqueous or water-solubleimpurities from the solvent-lights column 110 (via the aqueous stream124 purge).

The liquid side draw 222 sent to the decanter 120 may have a relativelysignificant amount of water and other water-soluble impurities such asmethanol. Thus, the decanter 120 may facilitate sufficient aqueousphase-out of the water and aqueous components on contact withhydrocarbon solvent. Therefore, solvent 128 (e.g., C8-C10) may beintroduced to the decanter 120 to promote formation of an aqueous phaseand an organic phase in the decanter 120. The organic phase in thedecanter 120 gives the organic stream 122, which may be sent as refluxto the solvents-lights column 110.

The aqueous phase in the decanter 120 gives the aqueous stream 124,which is sent to the water wash system 126, as discussed (see FIG. 7).This aqueous stream 124 may contain PO and also water, methanol,acetaldehyde, some methyl formate, glycol and other impurities. At thewater wash system 126, the aqueous stream 124 contacts additionalhydrocarbon solvent (e.g., C8-C10) and a relatively small amount ofwater to promote the removal of water soluble impurities such methanol,acetaldehyde, glycol, a relatively small amount of methyl formate, andother impurities via the aqueous purge 130 (FIG. 7) from the water washsystem 126. Propylene oxide (PO) is recovered in the solvent or organicphase returned in the wash organic stream 134 from the water wash system126 (FIG. 7) to the solvent-lights column 110 shown in FIG. 8. Thisrouting of the wash organic stream 134 is in contrast to the embodimentshown in FIG. 5, where the wash organic stream 134 from the water wash126 is instead sent to the decanter 120, and where the organics andrecovered PO reach the column 110 via organic stream 122 in FIG. 5.

In FIG. 8, an overhead stream 112, having light components, dischargesfrom the solvent-lights column 110 and is partially condensed in anoverhead condenser 114. In this example, the portion of the overheadstream 112 condensed is labeled as condensed overhead stream 118 whichis returned as reflux to the solvent-lights column 110. A vapor stream116 of non-condensed components is purged from the overhead condenser114. In certain embodiments with respect to FIG. 8, operation of theoverhead condenser 114 may be adjusted to give a vapor stream 116 purgein the range of 5-50 weight % range of the distillate (condensedoverhead stream 118) to give 60-90 weight % (e.g., about 75 weight %)total methyl formate purge from the crude PO feed 102.

A product stream having a majority of the PO entering the solvent-lightscolumn 110 in the crude PO stream 102 is discharged as a bottoms stream140 from the solvent-lights column 110. As discussed with respect toFIG. 7, the product stream (bottoms stream 140) is sent as feed to thedownstream solvent stripper 142 (see FIG. 7).

FIG. 9 reflects an exemplary solvent-lights column system 190-2 havingthe solvent-lights column 110 and a decanter 120 to facilitate formationand discharge of the aqueous stream 124. As similarly discussed withrespect to FIG. 8, the solvent-lights column 110 in the solvent-lightscolumn system 190-2 of FIG. 9 receives the crude PO stream 102. Asolvent 128 is also fed to the column 110. In the example of FIG. 9, thedecanter 120 is an overhead decanter and receives a condensed overheadstream 118 having the methanol, water, and other light aqueousimpurities, instead of receiving a side draw 222 (FIG. 8) having suchimpurities from the column 110.

In FIG. 9, an overhead stream 112, having light components, dischargesoverhead from the solvent-lights column 110 and is partially condensedin an overhead condenser 114. In this example, the condensed overheadstream 118 is sent to the decanter 120.

A vapor stream 116 of non-condensed components is purged from theoverhead condenser 114. In certain embodiments, operation of theoverhead condenser 114 may be adjusted to give a vapor stream 116 purgein the range of 5-50 weight % range of the distillate and to give 60-90weight % (e.g., about 75 weight %) total methyl formate purge from thecrude PO feed 102.

As with system 190-1 (FIG. 8), solvent 128 may be introduced in thesystem 190-2 of FIG. 9 to the decanter 120 to facilitate formation of anaqueous phase and an organic phase in the decanter 120. In thisillustrated example of FIG. 9, the organic stream 122 is returned asreflux to the column 110.

The aqueous phase discharges from the decanter 120 as aqueous stream 124to the water wash system 126 (see FIG. 7). As with system 190-1, thisaqueous stream 124 in system 190-2 generally contains PO and also water,methanol, acetaldehyde, some methyl formate, and other impurities. Theaqueous stream 124 is sent to the water wash system 126, contactingadditional hydrocarbon solvent (e.g., C8-C10) and a relatively smallamount of water, for ultimate removal of water soluble impurities ofmethanol, acetaldehyde, glycol and a relatively small portion of methylformate, and other impurities, via the downstream aqueous purge 130(FIG. 7). PO is recovered in the returning of the wash organic stream134 (PO and solvent) directly to the solvent-lights column 110 (not viathe decanter 120 as in FIG. 8).

A product stream (bottoms stream 140) having a majority of the POentering the solvent-lights column 110 in the crude PO stream 102 isdischarged as bottoms stream 140 from the solvent-lights column 110. Asdiscussed with respect to FIGS. 7 and 8, the product stream (bottomsstream 140) of FIG. 9 is sent as feed to the downstream solvent stripper142 (see FIG. 7). Lastly, it is noted that other configurations of thelights-solvent column system 190 are contemplated to form and dischargethe aqueous stream 124. In certain embodiments, a side cooler to thelights-solvent column 110, and/or other equipment may be employed, forexample.

FIG. 10 is an exemplary back-end 106B of a separation system 100 (FIG.4) associated with the front-end system 104B discussed above withrespect to FIGS. 7-9. As with the back-end 106A of FIG. 6, the back-end106B depicted in FIG. 10 includes an extraction column 160, solventcolumn 162, and PO column 164. For the sake of clarity, the respectivereboilers for each column 160, 162, 164 are not shown, and the overheadcondenser for the solvent column 162 is not depicted. The extractioncolumn overhead condenser 240 for the extraction column 160 and the POcolumn overhead condenser 242 for the PO column 164 are shown.

For the primary feed to the extraction column 160, condensed overheadfrom the upstream stripper column 142 (FIG. 7) is sent as PO stream 108Bto extraction column 160 for extraction with a solvent (e.g., C8-C10hydrocarbon). A source of the solvent for the extraction may be thesolvent bottoms stream 148 from the downstream solvent column 162. Ofcourse, other sources of extraction solvent may be employed.

An extraction overhead stream 168 from the extraction column 160 iscondensed in the extraction column overhead condenser 240, and a portionof the condensed extraction overhead stream 168 returned as reflux tothe extraction column 160. Another portion of the condensed extractionoverhead stream 168 is collected as distillate 199 and is sent to theupstream water wash system 126 (FIG. 7).

Advantageously, this purge of a portion of the condensed extractionoverhead stream 168 collected as distillate 199 to the water wash system126 generally contains formaldehyde, and thus reduces the amount offormaldehyde in the downstream PO column 164. Therefore, fouling ofequipment associated with any overhead lights purge (not shown) from thePO column 164 may be reduced. The fouling may be due to formaldehydepolymer formation, for example.

In certain embodiments, with the purge of distillate 199 of thecondensed extraction overhead stream 168 having the light componentformaldehyde, the need for an overhead lights purge (such as theoverhead lights purge 172 shown in FIG. 6) at the downstream PO column164 may be eliminated, as depicted in FIG. 10. Further, the PO in thedistillate 199 purge of condensed overhead stream 168 to the wash system126 may be recovered in the wash organic stream 134 from the wash system126 returned to the solvent-lights column 110 in the solvent-lightssystem 190 (see FIGS. 7-9). Moreover, the use of a PO product side draw(giving product side stream 248) in FIG. 10 reduces the amount of POleaving in the distillate 199.

An extraction bottoms stream 170 having solvent and impuritiesdischarges from the extraction column 160 and is fed to the solventcolumn 162. A hydrocarbon purge 178 (e.g., C6) is removed overhead and asolvent bottoms stream 148 is removed. As indicated, this solventbottoms stream 148 may be fed to the extraction column 160. In addition,take-off portions of the solvent bottoms stream 148, such as recyclesolvent 149 and back end solvent 151 in the illustrated embodiment ofFIG. 10, may be sent to unit operations in the front-end 104B (see FIG.7).

In this illustrated embodiment of FIG. 10, as indicated, a product sidestream 248, having the majority of PO entering the extraction column160, is discharged from the extraction column 160 to the PO column 164.This is in contrast to FIG. 6 where the product stream is the extractionoverhead stream 168.

In the PO column 164 of FIG. 10, a bottoms heavies purge 174 is removed.A PO column overhead stream 252 discharges overhead and is condensed inthe overhead condenser 242. An increased reflux rate of condensed POcolumn overhead stream 252 to the PO column 164 may reduce PO loss andbeneficially increase separation of propionaldehyde and acetone from thePO in the PO column 164, for example. The condensed portion of PO columnoverhead stream 252 collected as product distillate is labeled as POproduct stream 109B and may be analogous to the PO product stream 109 ofFIG. 4.

In one example, a separation system 100 (FIG. 4) having the front-end104B (FIGS. 7-9) and back-end 106B (FIG. 10) may give a relatively highyield of 98.5 weight % of PO recovery from the crude PO stream 102 inthe condensed PO column overhead stream 252 sent as distillate product.In that example, the PO product stream 109B has high purity of 99.98weight % PO with 10 ppm methyl formate. Lastly, it should be noted thatthe columns and associated equipment depicted in FIGS. 7-10 may becommercial scale, and have the sizing, internals, and materials ofconstruction discussed above.

FIGS. 11 and 12 are an alternate embodiment for a front-end 104C andback-end 106C, respectively, of a separation system 100 (FIG. 4). Thefront-end 104C of FIG. 11 has a lights column 260, a heavies column 262,and a solvent-lights column 265 (e.g., with the column “solvent” asC8-C10s). These column 260, 262, and 265 may each be a distillationcolumn. The associated back-end 106C of FIG. 12 has an extraction column269 (or also labeled a solvent-heavies column) and a solvent column 273.In this embodiment, the PO product stream 109C (analogous to PO productstream 109 of FIG. 4) discharges from the extraction column 269.Moreover, a solvent purge (extraction overhead stream 286) is sent fromthe overhead of the extraction column 269 to the front-end 104C, therebyreducing the amount of the impurity formaldehyde in the extractioncolumn 269 and PO product stream 109C.

For the sake of clarity, the respective reboilers and overheadcondensers for each of the columns in FIGS. 11 and 12 are not depicted,except for the solvent-lights column overhead condenser 267 associatedwith the solvent-lights column 265. Moreover, the front-end 104C of FIG.11 is depicted with the lights column 260 as receiving the crude POstream 102 and feeding the heavies column 262. However, this processorder within the front-end 104C may be altered (i.e., switched). Inother words, the front-end 104C may be configured such that the heaviescolumn 262 receives the crude PO stream 102 and feeds the lights column260. In either case, the feed to the third column, the solvent-lightscolumn 265, will typically be similar in composition and mass flow rate.

In the illustrated embodiment for the front-end 104C of FIG. 11, thecrude PO stream 102 is fed to the lights column 260 for removal of lightimpurities and hydrocarbons, for example C5, via a lights columnoverhead stream 264. The lights column bottoms stream 266 from thelights column 260 contains most or a majority of the PO entering in thecrude PO stream 102. This lights column bottoms stream 266 is fed to aheavies column 262 for removal of heavy components, water, somemethanol, and the like, via a heavies column bottoms stream 268 from theheavies column 262. Examples of removed heavy components in the heaviescolumn bottoms stream 268 can include propionaldehyde, acetone, and soforth.

The majority of the PO entering the heavies column 262 from the lightscolumn 260 discharges in a heavies column overhead stream 270 (a productstream) from the heavies column 266. This heavies column overhead stream270 has reduced methanol and water due to the presence of the upstreamlights column 260. The heavies column overhead stream 270 is fed to thesolvent-lights column 265. Further, a solvent (e.g., C6-C10s) isintroduced to the solvent-lights column 265 via all or a portion ofhydrocarbon bottoms stream 271 from the back-end 106C (FIG. 12). Asdiscussed below, this hydrocarbon bottoms stream 271 has solventrecycled from the back-end 106C.

At the solvent-lights column 265, the solvent-lights column bottomsstream 108C is a product stream containing a majority of the PO enteringthe solvent-lights column 110 (in heavies column overhead stream 270)from the upstream heavies column 262. This solvent-lights column bottomsstream 108C may be analogous to the PO stream 108 of FIG. 4.

As for impurities, the light and aqueous impurities discharge overheadfrom the solvent-lights column 265 in an overhead stream 274. Theoverhead stream 274 is condensed in an overhead condenser 267 to give acondensed overhead stream 275. A vapor purge 280 from the overheadcondenser 267 of about 5 to 10 weight %, for example, of the overheadstream 274 is maintained to remove some non-caustic and non-watersoluble light components, and so forth. A portion of the condensedoverhead stream 275 is refluxed to the solvent-lights column 265. Theremaining portion of the condensed overhead stream 275 (i.e.,distillate) is subjected to a caustic wash, as discussed below.

It should be noted that by operating the solvent-lights column 265 withsufficient solvent feed (e.g., via streams 270 and 271, and withtargeted management of the condensed overhead stream 275 reflux anddistillate, and so on, the light impurities of methyl formate,acetaldehyde, methanol, water, glycol, and the like, are generallyconcentrated in the condensed overhead stream 275 without an aqueousphase forming in the solvent-lights column 265 or in the condensedoverhead stream 275.

As mentioned, the portion of the condensed overhead stream 275 not usedas reflux but forwarded as distillate is sent to a caustic wash sectionand contacted with a slightly over-stoichiometric amount of caustic(e.g., sodium hydroxide), via caustic stream 282, equivalent to theamount of methyl formate in the portion of the condensed overhead stream275 forwarded as distillate to maintain a pH of 10-12 in the causticwash section in certain examples. In the illustrated embodiment, thecaustic wash section is the addition of the caustic via caustic stream282 and the mixer 284 that provides for mixing and residence time.

Further, a hydrocarbon solvent, such as an organic stream or solventfrom the back-end 106C system, may be introduced upstream of the mixer284, which may be, for example, a static mixer, to promote formation ofan aqueous phase (i.e., having a majority of the water, methanol,acetaldehyde, methyl formate, and other water-soluble impurities, in theportion of the condensed overhead stream 275 forwarded as distillate tothe caustic wash). The source of the solvent so added may be theextraction overhead stream 286 from the extraction column 269 in FIG.12. Moreover, this extraction overhead stream 286 may containformaldehyde and thus beneficially reduce formaldehyde in the back-end106C and ultimately in the final PO product 109C. Furthermore, the PO inthis extraction overhead stream 286 may be recovered in the recyclesolvent stream 278 sent as reflux to the solvent-lights column 265.

The caustic-treated distillate 288 from the mixer 284 is fed to abackwash column 290 which may be a relatively small PO wash/recovery,liquid-liquid extraction column having about 3-7 (e.g., 5) theoreticalstages satisfied via packing 296, for example, and generally in a middleportion of the column, for instance. Water 292 is introduced at a topportion and solvent 294 introduced at a bottom portion of the backwashcolumn 290. The extraction in the backwash column 290 increases POrecovery from the caustic/water waste in the caustic-treated distillate288 with reduced caustic carryover at reduced aqueous phase. In thisexample, the caustic/water waste 295 discharges from a bottom portion ofthe backwash column 290. The backwashed PO is returned to thesolvent-lights column 265 in an organic stream (recycle solvent stream278) as additional reflux for the column 265. Lastly, the solvent 294fed to the backwash column 290 is a solvent (e.g., C8-C10s) from theback-end 106C system (FIG. 12).

As discussed, FIG. 12 is the exemplary back-end 106C associated with theexemplary front-end 104C (FIG. 11) of an exemplary separation system 100(FIG. 4). Referring to FIG. 12, the extraction column 269 receives asfeed the product stream (solvent-lights column bottoms stream 108C) fromthe solvent-lights column 265 (FIG. 11). Further, the extraction column269 receives solvent 294 (e.g., C8-C10s) from a bottoms streamdischarging from the downstream solvent column 273.

A condensed extraction overhead stream 286 (condenser not shown) fromthe extraction column 269 is sent to the caustic wash section (mixer284, optionally a static mixer) of the front-end 104C (FIG. 11). Theextraction overhead stream 286 may generally have hydrocarbon solvent,formaldehyde, PO, and so on. The hydrocarbon solvent in extractionoverhead stream 286 may promote aqueous phase formation and separationin the backwash column 290 (FIG. 11). Further, the PO (e.g., 1-2 weight% based on the PO in the crude PO stream 102) sent in extractionoverhead stream 286 to the caustic wash (in mixer 284) and the backwashcolumn 290 may be recovered in the organic phase stream (recycle solventstream 278) sent as reflux to the solvent-lights column 265 (FIG. 11).Lastly, as discussed, this purging of formaldehyde with the extractionoverhead stream 286 may reduce the amount of formaldehyde in theextraction column 269 and in the PO product stream 109C discharging as aside draw from the extraction column 269.

Furthermore, a hydrocarbon bottoms stream 271, having heavy hydrocarbonsolvent (e.g., C6-C10s), for example, discharges from the extractioncolumn 269 and is fed to the solvent column 273. Moreover, a portion ofthis hydrocarbon bottoms stream 271 (e.g., having C6-C10s) may be fed tothe solvent-lights column 265 (FIG. 11). At the solvent column 273, ahydrocarbon purge 300 (e.g., having C6) is taken overhead. A solventstream 294 (e.g., having C8-C10s) discharges from the bottom of thesolvent column 273 and may be sent to the extraction column 269 for theextraction, and/or sent for the liquid-liquid extraction in the backwashcolumn 290 (FIG. 11).

Lastly, as indicated, the PO product stream 109C is recovered in a sidedraw (e.g., from the pasteurization section) of the extraction column269. In sum, an exemplary PO separation system 100 having the front-end104C and back-end 106C may provide an exemplary high yield of 98.9weight % (of the PO in the crude PO stream 102), for example, and anexemplary high purity of 99.99 weight % PO with 10 ppm methyl formate inthe PO product 109C.

The equipment depicted in FIGS. 11 and 12 may be commercial scale.Further, the respective diameters, heights, and the numbers oftheoretical stages of the various columns in FIGS. 11 and 12 may besized as a function of the design basis for the mass flow rate andcomposition of the incoming crude PO stream 102, for instance. Toprovide for the theoretical stages, trays or packing may be employed.Trays may include sieve trays, bubble cap trays or valve trays, and thelike. The packing, which may be structured or dumped, can be glass,metal, plastic, and ceramic, and so on. The metallurgy or materials ofconstruction of the various equipment in FIGS. 11 and 12 may includecarbon steel, stainless steel, fiberglass reinforced polymer (FRP),nickel alloys, and so on.

In summary, embodiments of the present techniques may provide for apropylene oxide separation system including a distillation column toreceive a crude propylene oxide stream, discharge an impurity streamhaving methanol and water, and discharge a bottoms stream having amajority of the propylene oxide entering in the crude propylene oxidestream. A decanter may receive the impurity stream and a hydrocarbonsolvent to provide for formation in the decanter of an organic phaseincluding propylene oxide and hydrocarbon solvent, and an aqueous phaseincluding a majority weight percent of the methanol and the waterentering in the impurity stream. A water wash system receives and purgesthe aqueous phase from the propylene oxide separation system, whereinthe organic phase in the decanter is sent to the distillation column.The crude propylene oxide stream may be a propylene oxide reactoreffluent stream, such as in a propylene oxide/tert-Butanol processsystem.

The distillation column may include an overhead condenser, and whereinthe distillation column is configured with an overhead vapor purge ofnon-condensed components from the overhead condenser. The decanter maybe an overhead decanter to the distillation column, and receive theimpurity stream from the overhead condenser. On the other hand, thedecanter is a side decanter to the distillation column, and isconfigured to receive the impurity stream from a liquid side draw of thedistillation column. The distillation column may be a solvent-lightscolumn. Further, the water wash system may include a static mixer and acoalescer. A solvent stripper may receive the bottoms stream from thedistillation column, wherein the solvent stripper discharges asolvent-stripper overhead stream having a majority of the propyleneoxide entering the solvent stripper in the bottoms stream from thedistillation column, and wherein the solvent stripper discharges asolvent-stripper bottoms stream having at least a portion of thehydrocarbon solvent received at the decanter. Lastly, an extractioncolumn may subject the solvent-stripper overhead stream from the solventstripper to a hydrocarbon solvent extraction to remove impurities,wherein the extraction column purges the removed impurities includingformaldehyde to the water wash system.

Embodiments may provide for a method of separating propylene oxide froma crude propylene oxide stream in a separation system, the methodincluding: feeding the crude propylene oxide stream to a distillationcolumn; discharging an impurity stream from the distillation column to adecanter, the impurity stream comprising methanol and water; feedinghydrocarbon solvent to the decanter; and forming in the decanter anorganic phase comprising propylene oxide and hydrocarbon solvent, and anaqueous phase comprising a majority weight percent of the methanol andthe water fed to the decanter in the impurity stream. Further, themethod may include washing the aqueous phase with water and purging thewashed aqueous phase from the separation system, and sending the organicphase to the distillation column.

The discharging of the impurity stream may include discharging theimpurity stream to the decanter via an overhead condenser of thedistillation column, and the method further including purging a vaporstream from the overhead condenser. On the other hand, the dischargingthe impurity stream may involve discharging the impurity stream to thedecanter via a liquid side draw of the distillation column. Lastly, themethod may include: discharging a bottoms stream from the distillationcolumn, the bottoms stream comprising a majority of the propylene oxideentering the distillation column in the crude propylene oxide stream;separating formaldehyde from the bottoms stream; and sending theformaldehyde to a water wash system performing the washing of theaqueous phase with water.

Certain embodiments may include a propylene oxide separation systemhaving a distillation column to receive a processed crude propyleneoxide stream, discharge an impurity stream comprising methanol andwater, and discharge a bottoms stream having a majority of the propyleneoxide entering in the processed crude propylene oxide stream. A mixermixes caustic (e.g., having sodium hydroxide) with the impurity streamto give a caustic-treated impurity stream. A backwash column subjectsthe caustic-treated impurity stream to both an aqueous extraction and anorganic extraction. The backwash column may purge an aqueous streamhaving a majority amount of the methanol and the water in the impuritystream. In addition, the backwash column may discharge an organic stream(having hydrocarbon solvent and propylene oxide) to the distillationcolumn. The propylene oxide separation system may include an extractioncolumn disposed downstream of the distillation column, and configured topurge formaldehyde to the mixer, wherein the formaldehyde is carryoverfrom the bottoms stream of the distillation column.

Lastly, some embodiments may include a method for separating impuritiesfrom propylene oxide, the method including processing via a distillationcolumn a propylene oxide stream to discharge an impurity stream havingmethanol and water, and to discharge a bottoms stream having a majorityof the propylene oxide entering the distillation column. The impuritystream is mixed (e.g., via a static mixer) with caustic (e.g., havingsodium hydroxide) to give a caustic-treated impurity stream which isthen extracted with hydrocarbon, and then extracted with water to purgean aqueous stream having a majority of the methanol and water in theimpurity stream. The method may include processing the bottoms streamfrom the distillation column and purging formaldehyde via the processingto the impurity stream.

Although the present invention has been described in considerable detailwith reference to certain preferred versions thereof, other versions arepossible. Therefore, the spirit and scope of the appended claims shouldnot be limited to the description of the preferred versions containedherein.

All the features disclosed in this specification (including anyaccompanying claims, abstract, and drawings) may be replaced byalternative features serving the same, equivalent or similar purpose,unless expressly stated otherwise. Thus, unless expressly statedotherwise, each feature disclosed is one example only of a genericseries of equivalent or similar features.

Any element in a claim that does not explicitly state “means for”performing a specified function, or “step for” performing a specificfunction, is not to be interpreted as a “means” or “step” clause asspecified in 35 U.S.C §112, sixth paragraph. In particular, the use of“step of” in the claims herein is not intended to invoke the provisionsof 35 U.S.C §112, sixth paragraph.

What is claimed is:
 1. A propylene oxide separation system comprising: adistillation column configured to receive a crude propylene oxidestream, discharge an impurity stream comprising methanol and water, anddischarge a bottoms stream comprising a majority of the propylene oxide;a decanter configured to receive at least a portion of the impuritystream and a hydrocarbon solvent to provide for formation in thedecanter of an organic phase comprising propylene oxide and hydrocarbonsolvent, and an aqueous phase comprising a majority weight percent ofthe methanol and the water entering in the at least a portion of theimpurity stream; a water wash system configured to receive and purge theaqueous phase from the propylene oxide separation system, wherein theorganic phase in the decanter is sent to the distillation column,wherein the decanter is a side decanter to the distillation column, andis configured to receive at least a portion of the impurity stream froma liquid side draw of the distillation column; a solvent stripperconfigured to receive the bottoms stream from the distillation column,wherein the solvent stripper is configured to discharge asolvent-stripper overhead stream comprising a majority of the propyleneoxide and; an extraction column configured to subject thesolvent-stripper overhead stream from the solvent stripper to ahydrocarbon solvent extraction to remove impurities, wherein the removedimpurities comprising one or more of formaldehyde, methyl formate,acetaldehyde and methanol are transferred to the water wash system. 2.The propylene oxide separation system of claim 1, wherein the crudepropylene oxide stream is a propylene oxide reactor effluent stream in apropylene oxide/tert-Butanol process system.
 3. The propylene oxideseparation system of claim 1, wherein the distillation column includesan overhead condenser, and wherein the distillation column is configuredwith an overhead vapor purge of non-condensed components from theoverhead condenser.
 4. The propylene oxide separation system of claim 1,wherein the distillation column is a solvent-lights column, and whereinthe water wash system comprises a static mixer and a coalescer.
 5. Thepropylene oxide separation system of claim 1, wherein the solventstripper is configured to discharge a solvent-stripper bottoms streamcomprising at least a portion of the hydrocarbon solvent received at thedecanter.
 6. The propylene oxide separation system of claim 1, furthercomprising: a mixer configured to mix caustic with at least a portion ofthe impurity stream to give a caustic-treated impurity stream; and abackwash column configured to subject the caustic-treated impuritystream to both an aqueous extraction and an organic extraction; whereinthe backwash column is configured to purge an aqueous stream comprisinga majority amount of the methanol and the water in the caustic-treatedimpurity stream and to discharge an organic stream comprisinghydrocarbon solvent and propylene oxide to the distillation column. 7.The propylene oxide separation system of claim 6, further comprising: alights distillation column configured to receive the crude propyleneoxide stream, remove light components, and discharge alights-distillation column bottoms stream comprising a majority of thepropylene oxide from the crude propylene oxide stream; and a heaviesdistillation column configured to receive the lights-distillation columnbottoms stream, remove heavy components, and discharge an overheadstream comprising a majority of the propylene oxide from thelights-distillation column bottoms stream, and wherein the overheadstream comprises a processed crude propylene oxide stream; wherein theprocessed crude propylene oxide stream forms the feed to thedistillation column.
 8. The propylene oxide separation system of claim6, further comprising: a heavies distillation column configured toreceive the crude propylene oxide stream, remove heavy components fromthe crude propylene oxide stream, and discharge an overhead streamcomprising a majority of the propylene oxide from the crude propyleneoxide stream; and a lights distillation column configured to receive theoverhead stream, remove heavy components from the overhead stream, anddischarge a lights-distillation column bottoms stream comprising amajority of the propylene oxide from the overhead stream, and whereinthe lights-distillation column bottoms stream comprises a processedcrude propylene oxide stream; wherein the processed crude propyleneoxide stream forms the feed to the distillation column.